Devuan is installed on two computers.

1. One desktop with one Audigi sound card.
Sound appeared in Firefox after deleting /etc/asound.conf .

2. The second laptop with an Intel sound card and HDMI output.
Accordingly, you need a /etc/asound.conf file indicating the desired output. Without it there is no sound at all, with it there is no sound only in Firefox.

3. If you apply plughw the sound disappears on both computers. In .xsession-errors instead

audio method: alsa

appears

audio method: null

Regards.

EDIT: note that the difference between "hw" and "plughw" is the absence/presence of audio format translations. "hw" tells ALSA to channel audio direclty to the card without translations while "plughw" tells it to insert audio format conversions as needed.

This is my /etc/asound.conf

defaults.pcm.card 0
defaults.ctl.card 0

pcm.!default {
  type hw
  card 0
             }

ctl.!default {
   type hw
  card 0
             }

There is no sound only in Forefox.

Yes, God bless him, the problem is not critical. Pure curiosity prompted me to start a thread about why it works on the same machine in one distribution and not in another.

Regards.

The Cubeb / OnMediaSinkAudioError lines are likely the relevant ones, I suggent a general websearch on those for a start. I don't have Debian / Devuan on any desktops right now to investigate myself.

What more does running with MOZ_LOG=cubeb:5 (or a log level of your choosing) have to say?
Do you have any custom ALSA configuration (.asoundrc, /etc/asound.conf)?

This is after adding

security.sandbox.content.write_path_whitelist	/dev/snd/

uly_e@aa:~/Desktop$ firefox
[GFX1-]: vaapitest: ERROR
[GFX1-]: vaapitest: VA-API test failed: failed to initialise VAAPI connection.

[Child 2236, MediaDecoderStateMachine #1] WARNING: 7f718262f4c0 OpenCubeb() failed to init cubeb: file ./dom/media/AudioStream.cpp:281
[Child 2236, MediaDecoderStateMachine #1] WARNING: Decoder=7f71936a0f00 [OnMediaSinkAudioError]: file ./dom/media/MediaDecoderStateMachine.cpp:4604
ATTENTION: default value of option mesa_glthread overridden by environment.
[GFX1-]: Couldn't sanitize RENDERER device: i915
ATTENTION: default value of option mesa_glthread overridden by environment.
ATTENTION: default value of option mesa_glthread overridden by environment.
^CExiting due to channel error.

No sound.
On the Debian forum I came across a message from Head_on_a_Stick that Firefox was built with alsa support. Maybe the latest versions don't include it?

Regards.

You'll probably need to visit about:config and set

security.sandbox.content.write_path_whitelist	/dev/snd/

And if you get e.g.

Sandbox: seccomp sandbox violation: pid xxxx, tid xxxx, syscall 16

in a terminal when you try to play audio, you'll need

security.sandbox.content.syscall_whitelist  16

Note this refers to native ALSA support, I don't know if it applies also to apulse but I wouldn't be at all surprised if it does.

On that: Devuan packagers, please build firefox with native ALSA support. It's literally as simple as a --configure option at compile time, and this apulse shenanigans is stupid and annoying.

  uly_e@AA:~/Desktop$ apulse firefox
[GFX1-]: vaapitest: ERROR
[GFX1-]: vaapitest: VA-API test failed: failed to initialise VAAPI connection.

[apulse] [error] do_connect_pcm: can't set sample format 14 ("FLOAT_LE") for playback device "default". Error code -22 (Invalid argument)
[apulse] [error] do_connect_pcm: failed to open ALSA device. Apulse does no resampling or format conversion, leaving that task to ALSA plugins. Ensure that selected device is capable of playing a particular sample format at a particular rate. They have to be supported by either hardware directly, or by "plug" and "dmix" ALSA plugins which will perform required conversions on CPU.
[Child 2836, MediaDecoderStateMachine #1] WARNING: 7f404cccc280 OpenCubeb() failed to init cubeb: file ./dom/media/AudioStream.cpp:281
[Child 2836, MediaDecoderStateMachine #1] WARNING: Decoder=7f405c2c9400 [OnMediaSinkAudioError]: file ./dom/media/MediaDecoderStateMachine.cpp:4604
ATTENTION: default value of option mesa_glthread overridden by environment.

uly_e@AA:~/Desktop$ export APULSE_PLAYBACK_DEVICE=plug:dmix apulse abrowser

uly_e@AA:~/Desktop$ apulse firefox
[GFX1-]: vaapitest: ERROR
[GFX1-]: vaapitest: VA-API test failed: failed to initialise VAAPI connection.

ATTENTION: default value of option mesa_glthread overridden by environment.
uly_e@AA:~/Desktop$

https://github.com/i-rinat/apulse/issues/85

Report about apulse in the middle of the thread.

Yes, I was affirming that as the solution, not pulsecrap or pipecrap.

/me doesn't often have to block an avatar . . .

I tried to squash that gnat 3 times before I figured out it was an avatar...

I copied the FF profile from openSuse to Devuan

That may be the problem. I did the same from one OS to another. Resolved it when I set the copied .mozilla directory to old and started fresh.

I been using uxp web browsers again for this  reason alone.

Not to mention, firefox is an ugly monster since 57+

The old UI looked fine, the new one is fisher price eye candy.

But I digress, apulse as was said, does help you to bypass this problem.

I don't know if you can use firejail with apulse though and still load it though.

I probably didn't convey the point well.
I repeat, on the same computer, with the same DE, the same version of FF in one OS, the sound works through ALSA, on the other it does not.

It's not a matter of DE, there is no sound in the failsafe session.
It's not a matter of settings, I copied the FF profile from openSuse to Devuan, the browser started with it, but again there was no sound.

In itself, this is not a problem for me, there are working alternatives to FF and install crutches in the form of pulse, etc. I won't.
  I'm interested in the fact itself

Regards.

I have alsa as well.